The solvent extraction process for recovering uranium consists of a sequence of chemical steps or operations performed on scrap material or spent fuel. First, the scrap material or spent fuel containing uranium compounds is treated with an aqueous solution of nitric acid (HNO.sub.3), whereby the uranium is dissolved to produce uranyl nitrate (UO.sub.2 (NO.sub.3).sub.2) and other acid-soluble components in an aqueous phase. This aqueous phase is passed down a solvent extraction column while an organic phase of tri-n-butyl phosphate (TBP) in an organic diluent of a paraffinic mixture, such as dodecane, is passed up through the extraction column in counter-current flow with the aqueous phase. The soluble uranium compounds of the aqueous phase are extracted therefrom by the organic phase and combined with the TBP. The uranium is thus separated from the acid-soluble raffinate contaminants remaining in the aqueous phase and carried by the organic phase from the extraction column. The aqueous and organic phases exit at opposite ends of the extraction column.
The organic phase effluent from the extraction column is then passed up through a stripping column while water is passed down through the stripping column in countercurrent flow with the organic phase. The water releases the uranium from the TBP of the organic phase, whereby it is transferred to and carried within the aqueous phase. The aqueous and organic phases exit at opposite ends of the stripping column, the aqueous phase containing the recovered uranium compounds separated from contaminants. The organic phase is then recycled back through the extraction column. Typically the procedure is carried out with a continuous flow of all components through the system comprising the extraction and stripping columns. The desired product of the solvent extraction process is a high-purity aqueous phase effluent containing virtually all the uranium of the initial waste fed into the system.
Ionizing radiation, elevated temperature and acids cause decomposition of TBP in all of the solvent extraction processes that separate heavy metals, such as uranium, plutonium, thorium and gadolinium. The decomposition products prevent separation of the desired products uranium and plutonium from unwanted wastes, such as gadolinium, zirconium, and fission products.
The acid feed to solvent extraction includes deesterification (dealkylation and hydrolysis) reactions producing dibutyl phosphoric acid (DBP), monobutyl phosphoric acid (MBP), phosphoric acid and butyl alcohol. The sequence of deesterification reactions are summarized below: ##STR1##
Dibutyl phosphoric acid (DBP) complexes with uranium, zirconium, and gadolinium, forming compounds soluble in TBP and its solution in hydrocarbon diluents (dodecane or kerosene). This results in a lower gadolinium or zirconium decontamination factor, and an increase in uranium losses in the raffinate, or aqueous waste, produced by the solvent extraction process.
Gas chromatography is the standard analytical method for determining the small quantities of DBP and MBP in TBP and TBP-diluent solutions. To measure these degradation products, DBP and MBP are converted to the more volatile methyl esters, namely, CH.sub.3 DBP and (CH.sub.3).sub.2 MBP.
Methylation prevents the problems of migration of the low-volatility acids through the packed columns of the gas chromatograph and of their dissociation within the columns. The methylating agent commonly used is diazomethane (CH.sub.2 N.sub.2), which is both explosive and toxic.